Dry polymers provide the advantage of lowest freight cost per pound of active polymer, but this advantage may be offset by the necessity to install, operate and maintain special polymer makedown systems.
Dry polymers are usually hygroscopic; they must be stored in dry conditions. Most suppliers ship dry polymers in a multiwall bag or fiber drum with a polyethylene liner to protect the polymer from moisture vapor. Larger air-tight stainless steel bulk shipping bins are also used. Polymer received in bags or fibre drums should be stored in an area that is beyond the reach of wash-up hoses and other sources of accidental wetting. Dry polymers kept dry have virtually unlimited shelf life. Different types of equipment are required for liquid, emulsion, and dry polymer makedown. It is necessary for each individual polymer molecule to be wetted in order to dissolve in water. Polymer ionizes in water, and, in doing so, the molecules uncoil as a result of the natural repulsion of similar charges along the length of the chain.
If dry particles are not individually wetted, clumps of undissolved polymer, often called fisheyes, will form. Besides the problem of plugging such things as ball check valves and filters, fisheyes represent unusable polymer molecules, decreasing the cost effectiveness of the application. Dry polymer makedown systems employ specially designed equipment for wetting the polymer molecules.
The first step in the use of a dry polymer is preparation of a polymer stock solution. The practical maximum concentration for this solution is limited to about 1% by viscosity buildup, stirring capability, and pumping and piping considerations. Initial polymer solution preparation is almost always a batch operation. The equipment required for dissolving dry polymer usually consists of a wet-out device (dry solids eductor or other special polymer wetting equipment) and a dissolving tank with a low shear agitator. In most cases, all of this equipment must be made of corrosion resistant materials such as stainless steel or plastics.
The key to dissolution of a dry polymer is achieving uniform wet-out of each polymer particle followed by subsequent rapid dispersion. If uniform wet-out is not achieved, large gelled lumps of polymer will be formed; these gels are extremely difficult to dissolve. In a small mill, uniform polymer wet out may be achieved by slowly sprinkling the dry polymer on the sides of the vortex of a rapidly stirred tank of water. In larger mills, it is usually necessary to use a dry solids eductor specially designed for this purpose. The largest users of dry polymers usually automate this unit operation with a device that sprinkles polymer particles onto water flowing through a wire. After the necessary amount of polymer for a batch is added, the desired amount of additional water is run into the tank and stirring is continued until all of the polymer is dissolved and a uniform solution is achieved. This usually requires a 1 to 2 hour stirring period. After dissolution is complete, agitation is stopped, and the solution is pumped to the storage or feed tank. Before it reaches the storage tank, the solution should be gravity screened through a fine mesh wire strainer to remove undissolved polymer and gel particles. The shelf life of polymer stock solution varies by product and ranges from a few days to 2 months.